Discovery of Most Distant Supernovas -- Indicators of the Fate of the Universe

January 16, 1996

Lynn Yarris,

With their discovery of the most distant supernovas (exploding stars) ever observed, an international scientific team led by researchers with the Ernest Orlando Lawrence Berkeley National Laboratory may be on the verge of learning the ultimate fate of our universe. The work is being presented today at the American Astronomical Society meeting in San Antonio, TX.

In all, 18 supernovas were discovered by the brilliant light of their self-destruction: a supernova explosion can be brighter than the entire galaxy of billions of stars in which it is born. Eleven of them, including several of the most distant known stars, were found at the end of November within one 48 hour period. This record number of discoveries at record distances is dramatic proof of the technique developed at the Berkeley Lab to make deep space supernova discoveries possible, and eventually even routine.

The supernovas were found in galaxies ranging from four to seven billion light years away. (A light year is the distance light can travel in a single year -- approximately 6 trillion miles.) This means that the furthest of these explosions took place billions of years before our own solar system was formed, halfway back to the beginning of the universe. Only now is the light reaching Earth.

The discoveries were made as part of the "Supernova Cosmology Project" which is headed by Saul Perlmutter, an astrophysicist in the Berkeley Lab's Physics Division. The goal of the project is to measure the universe's "deceleration" -- the rate at which the known continual expansion of the universe is slowing down. At stake is an understanding of the way the universe will end: having begun expanding with the Big Bang, will it continue to expand forever, becoming ever more cold and empty, or will it slow to a halt, turn around, and eventually contract to a fiery dense finale -- a Big Crunch.

"Distant supernovas provide natural milemarkers useful in determining these trends in the cosmic expansion," says Perlmutter. The redshift of the distant supernovas -- a shift in the colors of their spectra's features -- tells how much the universe has expanded during the billions of years that the light has travelled from the supernova to our telescopes here on Earth. If this total amount of expansion is more than expected from the current expansion rate -- called the "Hubble constant" -- then the universe must have been expanding faster in the past and be slowing down. And if it is slowing enough, it will eventually come to a halt and start to contract.

Conversely, the project could discover that the expansion is actually accelerating. This scenario has recently been proposed by a number of scientists, as a solution to discrepancies between the ages of the oldest stars and the recent measurements of the Hubble constant.

Today's presentation at the AAS meeting, based on their first seven supernova discoveries, already provides intriguing hints of deceleration. Perlmutter and his colleagues on the Supernova Cosmology Project are now tracking the brightness of the recently discovered supernovas. Once these observations have been completed, the first determination of the deceleration of the universe will follow shortly. As the project continues to discover supernovas over the next few years, the accuracy of the measurement will continue to improve.

The measurement technique is based on the fact that most of the distant supernovas are classified as "Type Ia," the brightest type of supernova. "Type Ia supernovas can serve as a measurement of distance because they can be calibrated as a standard candle with a known brightness," says Perlmutter. "By measuring the light reaching us from a distant Type Ia supernova and comparing this to the known brightness of similar supernovas in nearby galaxies, we can calculate the supernova's distance from earth."

The technique used to discover the 11 recent supernovas of 1995 is the same that was used by the Supernova Cosmology Project team to discover the seven supernovas of 1994 and 1993. An ultrasensitive electronic camera attached to a telescope is used to photograph thousands of deep-space galaxies at the time of a new moon. A second set of images of those same galaxies is taken at the same telescope just before the next new moon. Using some of the most powerful desktop computers, the two sets of images are compared and light from the older image is subtracted from new image light to reveal the appearance of supernovas.

"A Type Ia supernova can shine almost as brightly as an entire galaxy, but only for about a month before it becomes too faint for even the largest telescopes to observe," says Gerson Goldhaber, a Berkeley Lab and University of California, Berkeley professor and one of the key members of the project. "For the purpose of measuring distances, it is important that we are discovering Type Ia supernovas just before or at their brightest moments."

The experiment has only recently become possible, since it depends on the advances in light-detectors, computers, and the Internet, which ties together the most important ingredients: astronomers using the newest and largest telescopes around the world. All of the recent supernovas were sighted on the Cerro Tololo Inter-American Observatory 4-meter (158-inch) telescope in Chile, the largest in the southern hemisphere. Brightness measurements of the supernovas were collected by Di Harmor, Daryl Willmark, and Dave Silva at the recently commissioned WIYN telescope on Kitt Peak, AZ, by Heidi Newberg at the recently commissioned ARC telescope, at Apache Point, NM, by Richard McMahon, Mike Irwin, Ariel Goobar, and Dave Carter at the 4-meter and 2.5-meter telescopes on the Canary Islands (off Africa), and by Warrick Couch and Richard Ellis at the Anglo-Australian 4-meter telescope in Australia. It was the world's largest telescope, the Keck 10-meter (400-inch) Telescope in Hawaii, that provided the observing power to identify the supernova spectra -- observations made with University of California, Berkeley astronomers Alex Filippenko, Aaron Barth, and Berkeley Lab's Isobel Hook. The Keck Ten Meter Telescope was designed at the Berkeley Lab specifically for the observation of faint distant objects such as these supernovas.

An unusual aspect of this project is that high-school students around the country can participate over the Internet in the search for supernovas, sharing the data with the researchers as it is collected at the telescopes. The students use simplified analysis software developed by the educational arm of the Berkeley Lab team, called the "Hands-On Universe" program. In principle, the students could find the supernovas before the scientists do. So far, schools in Chicago and Massachusetts are beginning to work with these data, in the pilot stages of this novel coordination of education and forefront research.

The Central Bureau for Astronomical Telegrams sent an International Astronomical Union telegram on December 6 announcing the recent supernova discoveries to observers around the world. The participants in the discoveries listed on the telegram were: S. Perlmutter, S. Deustua, G. Goldhaber, D. Groom, I. Hook, A. Kim, M. Kim, J. Lee, J. Melbourne, C. Pennypacker, and I. Small, Lawrence Berkeley Lab. and the Center for Particle Astrophysics; A. Goobar, Univ. of Stockholm; R. Pain, CNRS, Paris; R. Ellis and R. McMahon, Inst. of Astronomy, Cambridge; and B. Boyle, P. Bunclark, D. Carter, and M. Irwin, Royal Greenwich Obs.; with A. V. Filippenko and A. Barth (Univ. of California, Berkeley) at the Keck telescope; W. Couch (Univ. of N.S.W.) and M. Dopita and J. Mould (Mt. Stromlo and Siding Spring Obs.) at the Siding Spring 2.3-m telescope; H. Newberg (Fermi National Accelerator Lab.) and D. York (Univ. of Chicago); D. Harmor, D. Willmark, and D. Silva at the WIYN telescope; and A. Walker, at CTIO.

The Supernova Cosmology Project is jointly sponsored by Berkeley Lab and the National Science Foundation's Center for Particle Astrophysics at the University of California, Berkeley. The project is also affiliated with the new Institute for Nuclear and Particle Astrophysics at the Berkeley Lab.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

For More Information: contact Dr. Saul Perlmutter (510) 486-5203